Conventionally, on a TFT array substrate being a board device to be used for, for example, a liquid crystal display device, a silicon nitride film (SiN film) that serves as an insulating film may be formed on an ITO (Indium Tin Oxide) film being the oxide conductive film that becomes an electrode. Such a TFT array substrate is subjected to heating and film formation in a vacuum state by using a plasma CVD apparatus.
Generally, when forming a nitride film, it is possible that a mixed gas mainly composed of mono-silane (SiH4) gas and ammonia (NH3) gas is used as a material and a nitride film is formed with a plasma CVD apparatus.
When forming a silicon nitride film on the ITO film, in a deposition chamber of a plasma CVD apparatus, successively after gas pressure regulation, a silicon nitride film is formed. A substrate temperature at this time is preferably in the range from 200° C. to 450° C. for obtaining excellent film quality.
Through this film formation using a plasma CVD apparatus, a highly reliable silicon nitride film can be comparatively easily and efficiently manufactured.
However, in the case where a silicon nitride film is formed on the ITO film by using a plasma CVD apparatus, adhesion between the formed silicon nitride film and the ITO film is weak, and the formed film may easily peel. A possible cause of this lowering in adhesion is that plasma degradation of the silane gas and ammonia gas being material gases lowers adhesion between the ITO film surface and the silicon nitride film.
As a method for improving the adhesion between the ITO film and the silicon nitride film formed on the ITO film, a method in which an oxide film (SiOx film) is formed under the silicon nitride film is known. However, the ITO film or the insulating film layer (flattening film) being a layer under the ITO film is weak in terms of moisture, so that formation of an oxide film may cause a reliability failure. Therefore, the method in which an oxide film is formed under the silicon nitride film is inappropriate.
In addition, in the method in which washing is performed as pretreatment before formation of the silicon nitride film, the insulating film layer that is a layer under the ITO film may absorb moisture and causes the same failure as described above, so that this method is also inappropriate.
Therefore, for keeping adhesion between the ITO film and the silicon nitride film on the ITO film, a method is known in which the film stress of the silicon nitride film is adjusted to avoid peeling of the silicon nitride film. In particular, by using this film stress as a tensile-side stress, peeling of the silicon nitride film can be suppressed.
However, it is not easy to continuously adjust the film stress of the silicon nitride film, and control of the film thickness and stress according to the design of a product is troublesome.